Fillers are added to a papermaking pulp to fill void spaces not occupied with the fibres and thus to smoothen the surface of paper. They improve for example paper printability, dimensional stability, formation, and gloss. Added to this, optical paper properties like opacity, light scattering, and brightness are usually improved, because fillers' light scattering coefficient and brightness are often higher than those of pulp.
Fillers are low-priced when comparing to wood fibers and thus also used in a paper manufacture to reduce the costs of papermaking raw materials. Also drying of the filler-bearing paper web requires less energy. In spite of their inexpensive price and positive effects to paper properties, fillers have also negative features. They interfere inter-fiber bonding by adsorption or precipitating on fiber surfaces. Because of this, paper tensile strength and tensile stiffness are reduced and in printing there can appear linting. Also abrasion on paper machine can increase because of fillers. Their retention is quite poor too and can cause two-sidedness on paper.
In packaging board grades, fillers are not typically used or used in a very low amounts compared to other paper grades. Typical reasons for this are that they increase weight of the board without giving strength properties and that they reduce calibre in the same grammage. Calibre is most important parameter for bending stiffness. Also the fillers reduce elastic modulus, which is an important parameter for bending stiffness.
Bleached pulp is quite often used in the top ply of the board. Target with this is to have higher brightness and generally improved appearance of the board. Even on such cases only very low filler amounts are used and typically quite expensive fillers, such as TiO2, calcined kaolin, etc., are used to optimize elastic modulus of the top ply and maximize board bending stiffness. Quite often top ply grammage is optimized against whiteness and brightness and visual appearance instead of optimizing it against maximal bending stiffness.
Thus there would be a high need to improve whiteness and opacity of the board top ply while maintaining board bending stiffness and at the same time use low cost fillers.
One quite typical filler used in paper making is precipitated calcium carbonate (PCC). Typically the production of PCC has been produced separately from the actual papermaking process. PCC is normally produced at a dedicated plant located close the paper mill.
In WO 2011110744 A1 a method and a reactor for in-line production of calcium carbonate (PCC) in connection with the production process of a fibrous web is disclosed. This relates to in-line production of PCC into a suspension to be used in the production of the fibrous web, especially preferably directly into the flow of fibrous pulp, one of its partial pulp flows or a filtrate flow used in the production of fibrous pulp. This method has several advantages as reduced investment costs, since there is no need to have a separate PCC plant. Further there is a reduced need of retention chemicals as PCC is at least partially precipitated directly onto fibres.
In EP2287398A1 a method for obtaining a calcium carbonate, possibly fibers and fiber-fibril containing composite is obtained in which the calcium carbonate particles, if needed with the fibrils and fibers are connected, which is characterized by good dewatering capability and which for the manufacture of paper with a large amount of filler, with a great strength and having a large specific volume. This invention is achieved by the combination of five measures, the use of specific calcium carbonate particles, which is (d50) and has a scalenohedral morphology and an average particle diameter of more than 2.5 μm and a maximum of 4 μm micrometers, by the setting of a weight ratio of fibrils to calcium carbonate particles in the suspension before the coprecipitation of 0.2:1 to 4:1, by the use of fiber-fibrils and trough the setting of a weight ratio of calcium ions into the fibrils before the coprecipitation of 0.02:1 to 0.2:1. However this method describes a traditional off-line precipitated calcium carbonate using carbon dioxide and lime milk process with fibers.
There is thus a need for a new process for the production of a board ply having a desirable visual appearance, but also an optimized elastic modulus.